The rotating blades of low-pressure steam turbines induce tremendous centrifugal forces into the rotor. This can be a limiting factor in designing the turbine for maximum efficiency. A solution is to use lower density blade materials as such blades exert less force into the rotor. This solution can, however, only be applied if the low-density material has adequate mechanical properties. While Titanium is one existing solution, in some circumstances it may be preferable to use alternatives with even better strength to weight ratios. Another alternative is composite materials, an example of which is disclosed in Swiss Patent Number CH547943.
A disadvantage of composite materials is that they typically have less temperature tolerance than metals. This can be a problem, in particular during low volume flow operation and full speed conditions when last stage blades are susceptible to windage heating of the blade tip area. Normal blade temperatures typically do not exceed 65° C. However, without corrective means, due to this condition, last stage blade tip temperatures can exceed 250° C. At such temperatures, the mechanical properties of composite material are significantly impacted and they may undergo permanent degradation including deformation and reduced strength.
A solution to windage heating is provided by Patent application No. US2007/292265 A1. The solution comprises injecting a cooling medium in the vicinity of the last stage tip region. The medium, which includes either steam or water, may be injected from the casing either fore or aft of the blade tip. As an alternative, or in addition, a small extraction groove for extracting flow through the outer sidewall may be provided near the blade tip just forward of the blade.
Typically these control measures are taken based on predictive assessment using means such as computational fluid dynamic calculation methods. As the optimum control measure typical varies between installations and further with operating conditions, simple universally applied threshold values are likely to lead to suboptimal control. In particular, the control action may inject more steam/water than is actually required. This may lead to excessive erosive attack, in particularly along the blades leading or trailing edges, dependent on how and where the injection occurs. In other case, the control action may not inject enough steam/water, resulting in inadequate cooling. In yet further cases, too much working fluid may be extracted via extraction grooves detrimentally affect steam turbine efficiency. It is therefore desirable to provide a control system that ensures acceptable blade life while maximising turbine efficiency and minimising the consumption of the cooling fluid.